Basic question, I know, but other than an open circuit due to a
winding burnout, or maybe a shorted turn, how else can a transformer
fail?

Is it likely/possible that the primary voltage of a mains step-down
transformer can appear on the secondary due to some fault?

I have a transformer that has an oval core encapsulated in plasic with
the primary on one side of the core and the secondary on the opposite
side. Other than a simple open or shorted turn, the possibility of
mains voltage getting to the secondary must be impossible due to the
physical separation, right?

Basic question, I know, but other than an open circuit due to a
winding burnout, or maybe a shorted turn, how else can a transformer
fail?

Is it likely/possible that the primary voltage of a mains step-down
transformer can appear on the secondary due to some fault?

I have a transformer that has an oval core encapsulated in plasic with
the primary on one side of the core and the secondary on the opposite
side. Other than a simple open or shorted turn, the possibility of
mains voltage getting to the secondary must be impossible due to the
physical separation, right?

Click to expand...

Maybe.

Ferinstance, what happens if primary shorts to the core, and secondary
is ALSO shorted to the core? without actually trying it, it seems fairly
intuitive that the primary voltage *COULD* appear on the secondary. Not
"will", but "could"...

Basic question, I know, but other than an open circuit due to a
winding burnout, or maybe a shorted turn, how else can a transformer
fail?

Is it likely/possible that the primary voltage of a mains step-down
transformer can appear on the secondary due to some fault?

I have a transformer that has an oval core encapsulated in plasic with
the primary on one side of the core and the secondary on the opposite
side. Other than a simple open or shorted turn, the possibility of
mains voltage getting to the secondary must be impossible due to the
physical separation, right?

I have seen mains transformers of this type in only a few items of
equipment. Usually more expensive than other more common types but P -
S isolation is absolute and makes them very safe for medical
equipment.

Ferinstance, what happens if primary shorts to the core, and secondary
is ALSO shorted to the core? without actually trying it, it seems fairly
intuitive that the primary voltage *COULD* appear on the secondary. Not
"will", but "could"...

I have seen mains transformers of this type in only a few items of
equipment. Usually more expensive than other more common types but P -
S isolation is absolute and makes them very safe for medical
equipment.

Click to expand...

** But *no way* are the primaries and secondaries on opposite sides of the
R core !!!

** But *no way* are the primaries and secondaries on opposite sides of
the R core !!!

There are two of each, wound side by side & interconnected.

Click to expand...

These appear to be C-cores, which are similar to toroids, having a
tape-wound construction that is more efficient than E-I laminations. Unlike
toroids, however, C-cores are generally cut so the winding bobbins can be
installed, and then held back together, but this causes gaps in the
laminations which cause higher impedance. Some high voltage transformers
are made this way, with primary and secondary on opposite sides of the
core.

The safest transformer design I have seen is that used by Signal
Transformer 241 series, Stancor SWC series, and and Triad F4-F8 series, as
well as PC mounted styles like Tamura 3FS series. They are standard EI
cores, but the bobbins are molded plastic with a divider, so the primary
and secondary windings are physically isolated from each other.

Usually a transformer winding will fail open in the hot spot deepest in the
windings, or else where it is connected to the leads. I have seen most fail
in the primary, where the insulation breaks down between two or more
windings, causing a high current in that small area, which melts the wire.

Some smaller transformers are impedance protected, or power limited, so
they will not fail catastrophically with an overload or shorted secondary.
I think this is how most wall-warts are designed. It is always good
practice to ground the core of power transformers, so any primary to core
short will not cause dangerous currents in the secondary. Larger
transformers should be protected with fuses or circuit breakers rated
according to normal current draw.

** But *no way* are the primaries and secondaries on opposite sides of the
R core !!!

There are two of each, wound side by side & interconnected.

....... Phil

Click to expand...

Just looked at the one I have. Yes, wound as you mention. The plastic cores
are half shells that clip together and the whole thing looks much more
effort to make than a normal toroid.
Got mine (20Vac) from a cheapo mass produced hifi. Somehow just didn't seem
right that it was in there. Couldn't bear to throw it away as the shaped
tape core is a work of art.

Ferinstance, what happens if primary shorts to the core, and secondary
is ALSO shorted to the core? without actually trying it, it seems fairly
intuitive that the primary voltage *COULD* appear on the secondary. Not
"will", but "could"...

Click to expand...

I can't immagine a scenario in which that is more likely than the primary
shorting to the secondary. (or vice versa)

** But *no way* are the primaries and secondaries on opposite sides of the
R core !!!

Click to expand...

Did you not see this paragraph on the link I gave.

"The other unique feature is the use of Bobbins in two parts. The
primary winding and secondary winding are done on separate bobbins
thus ensuring complete isolation between the two windings. This allows
meeting of any safety standards requirement."

"The other unique feature is the use of Bobbins in two parts. The
primary winding and secondary winding are done on separate bobbins
thus ensuring complete isolation between the two windings. This allows
meeting of any safety standards requirement."

Click to expand...

I think I get what you are referring to Phil.

You are correct in that the windings are distributed over both
bobbins. The windings on an individual bobbin are separated by an
interleaving barrier of the same material as the main bobbin.

That is NOT the page I gave though. You have gone to the comparisons
page and I didn't give that reference. I also don't consider that the
term "double-structure bobbin" conveys accurately to those not
familiar with transformer winding how the separation is achieved. I
gave the main page link so that anyone wanting to read further could
do so, not to start a war on specific details.

( ie, the fact the P and S windings are duplicated and interconnected )

** Shame how the specific details of the winding arrangement IS the point
at issue.

Shame how YOU quoted a para on the main page that was ambiguous on the
point at issue.

Shame how YOU did not read the site further to resolve that ambiguity.

Shame how YOU treated an ambiguous marketing blurb as technical fact.

Shame how you did not have the sense to realise that any transformer with P
and S winding on separate limbs of a core would have very poor voltage
regulation.

Shame, shame shame,

....... Phil

Click to expand...

My original post was simply to ask the OP if he was in fact referring
to an R-Core transformer. Somehow you seem to want to blow the whole
thing into a discussion on the specific technicalities of how an
R-Core transformer is constructed. While the page I gave didn't give
all the details in this regard I didn't want to get into the specifics
of the issue. Manufacturers will naturally slant advertising to list
what they perceive as advantages for their products and I admit I
didn't give too much thought to the content of the page to determine
what it really said. And I admit that the ambiguity of the statement
on the page did mis-lead me to some extent. However, I don't see it as
any shame on myself for not delving into technical complexities
considering that I was mainly asking the OP if he was referring to an
R-Core in my first post.

My original post was simply to ask the OP if he was in fact referring
to an R-Core transformer.

Click to expand...

** Bollocks.

Your initial post contained a serious error that needed to be corrected

So I politely did so.

Somehow....

Click to expand...

** No "somehow" about it - see above.

you seem to want to blow the whole
thing into a discussion on the specific technicalities of how an
R-Core transformer is constructed.

Click to expand...

** Naturally - in order to correct the error YOU posted then later tried
to fallaciously prove was right.

This one:

" I have seen mains transformers of this type in only a few items of
equipment. Usually more expensive than other more common types but P -
S isolation is absolute and makes them very safe for medical
equipment."

Grow up - Ross.

While the page I gave didn't give
all the details in this regard I didn't want to get into the specifics
of the issue.

Click to expand...

** That is a damn lie.

Grow up anytime - Ross.

Manufacturers will naturally slant advertising to list
what they perceive as advantages for their products and I admit I
didn't give too much thought to the content of the page to determine
what it really said.

Click to expand...

** No sign of YOU thinking the matter through at all.

Grow up anytime - Ross.

And I admit that the ambiguity of the statement
on the page did mis-lead me to some extent.

Click to expand...

** And you posted that false impression in contradiction of my earnest
attempts to correct your original error.

However, I don't see it as
any shame on myself for not delving into technical complexities

Click to expand...

** Bollocks.

You were wrong, stubborn and rude about it too.

Grow up anytime & take responsibility for your actions.

considering that I was mainly asking the OP if he was referring to an
R-Core in my first post.

Click to expand...

** How typically dishonest of YOU to ignore and want others to ignore
your posting of serious misinformation.

Piss off and do something useful - Ross.

Like checking out those illegal 2 core IEC leads that are in nearly every
hobby shop in Perth and reporting them to that fellow in the dept.

Want to reply to this thread or ask your own question?You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.